It sounds more like sleep agression than people agression. Many greyhounds will snap or bite if they're startled awake. I had a good old boy for about 10 years, and like you said he was bombproof. In bed one night I inadvertently landed on his head with my own, and he bit me really hard. He didn't draw blood, but it hurt so much that I actually cried. I didn't blame him at all, it was totally my fault.
I have another boy whom I've had for a few years, and he slept in bed with me. One night I moved and bumped him, and he bit me. I immediately kicked him off of the bed, and he wasn't allowed back for months until he learned that I owned the bed, not him.
I know it's upsetting to be bitten by your dog, but don't give up on her. As long as she's not generally agressive towards you, just be careful approaching her if she's lying down or sleeping. Make sure she's awake and knows you're going to touch her before you do. You might also want to limit her access to the bed for a while until you feel you can trust her again.